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Moon and Aldebaran

StarDate:

November 21, 2010

The tip of your little finger held at arm’s length is more than big enough to cover up the Sun. But that wouldn’t be the case if the Sun were the size of Aldebaran, the bright orange star that represents the eye of Taurus, the bull. Aldebaran is more than 40 times wider than the Sun. So if it took the Sun’s place, it would extend half-way out to the orbit of Mercury.

There are several ways to measure a star’s size. All of them require a good measurement of the star’s distance from Earth. It’s easiest to get good distances for stars that are close -- like Aldebaran, which is just 67 light-years away.

Because Aldebaran is big and close, astronomers can directly measure its size in the sky. The size of that angle, combined with the star’s distance, tells us the star’s true size.

Aldebaran also lies along the Sun’s path across the sky. The Moon follows this path, too, so it sometimes covers up Aldebaran. Measuring how long it takes the star to disappear behind the Moon also reveals its size.

And by measuring the sizes of many stars, astronomers have found that there’s a relationship between a star’s size and its temperature and total energy output. So measuring the temperature and brightness of any star -- which also requires a good estimate of its distance -- also reveals its size.

Look for giant Aldebaran rising below the Moon early this evening, and keeping a close eye on the Moon throughout the night.

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